1. Field of the Invention
The present invention relates to an electronic watermark embedding method and the like for embedding an electronic watermark to control the duplication of digital information contents.
2. Description of the Prior Art
Nowadays, digital television broadcast has been realized, so that the delivery of an image through an image digital signal is widely being performed. Moreover, there is being spread also a video recorder for recording an image as digital data onto an optical disc and a magnetic tape. If an image can be recorded as the digital data, it is possible to duplicate an image without deteriorating the quality of the image. Accordingly, from the viewpoint of copyright, it is necessary to limit the duplication of the image.
For example, European Patent Application EP1006722A2 (corresponding to Japanese Patent Application Laid-open No. 2000-173175) discloses a watermark (electronic watermark) technique as one of techniques to limit the duplication of the image by embedding information regarding the duplication limit in the digital image. The watermark is embedded so as to be hidden in the image. Accordingly, the watermark embedded in the image hardly lowers the image quality. Moreover, a person who has received the image having the watermark embedded probably cannot recognize that the watermark is present in the image, and accordingly, it becomes very difficult to remove the watermark from the image.
The aforementioned EP1006722A2 discloses a system using a watermark indicating the prohibition of the duplication and a watermark indicating the allowance of one-generation duplication. Such a system can control the number of times that the duplication is performed, and accordingly, the system does not cause a copyright problem. Further, a person who receives television broadcast can watch a delivered image and without being restricted to a broadcast time.
In this system, when a digital image is supplied to a video recorder, the video recorder checks whether the image has a watermark. If one is present and it indicates to prohibit the duplication, the video recorder does not record the image. Moreover, when no watermark is present or a watermark is present but it indicates the allowance of one-generation duplication, the video recorder decides that the image is allowed to be duplicated and records the image onto an optical disc.
When an image is broadcast from a broadcast station, normally, a plurality of image contents are continuously broadcast. Accordingly, a variety of image contents, each having different image duplication control information, i.e., duplication prohibition, one-generation duplication allowance, duplication allowance, are successively broadcast. For example, the image contents having duplication control information indicating the duplication prohibition may be followed by the image contents of having duplication control information indicating the duplication allowance.
However, check to determine whether a watermark is present and detection of the watermark type switching from one to another require a predetermined period of time (for example, a predetermined time not greater than 10 seconds). Therefore, the image contents switching may not be detected rapidly enough and at a head portion of the subsequent image contents, control information different from an actual one may be recognized.
For example, in a reception apparatus and a reproduction apparatus, there is caused a trouble of mismatching between control information based on a watermark and actual control information in a sector. This disturbs, for example, control of a micro vision signal as copy (duplication) control information to be added to an analog video signal. Moreover, a recording apparatus also has a problem. For example, even after the image contents are switched from those having control information of the duplication prohibition to those having control information of the duplication allowance, a head portion of the subsequent image contents is recognized as in the duplication prohibition and cannot be recorded. Moreover, even if the contents having control information of one-generation duplication allowance are switched to contents having control information of duplication allowance, a watermark of a duplicated state may be embedded to a head portion of the subsequent contents, and then the contents are scrambled. On the contrary, there is a trouble that a head portion of the contents whose duplication is prohibited is recorded.